1. Technical Field
The present invention relates to separators constituting a polymer electrolyte fuel cell and relates to a fuel cell using such separators.
2. Related Art
In a polymer electrolyte fuel cell, a separator is layered on both sides of a plate-shaped membrane electrode assembly to form a unit of the layered structure, and a plurality of the units are layered to form a fuel cell stack. The membrane electrode assembly has a three-layered structure, in which an electrolytic membrane made from a resin such as ion-exchange resin is interpolated between a pair of gas diffusion electrode plates (positive electrode plate and negative electrode plate). The separator is stacked in a manner so as to contact the gas diffusion electrode plates of the membrane electrode assembly with gas passages for flowing a gas and cooling medium passages being formed between it and the gas diffusion electrode plate. According to this fuel cell, for example, hydrogen gas as a fuel is supplied to the gas passages facing the gas diffusion electrode plate at the negative electrode side, and an oxidizing gas such as oxygen or air is supplied to the gas passages facing the gas diffusion electrode plate at the positive electrode side, whereby electricity is generated by electrochemical reaction.
The above-mentioned separator must have functions for supplying electrons generated through the catalyst reaction of the gas (hydrogen gas) on the negative electrode side to an external circuit, and also for supplying electrons from the external circuit to the positive electrode side. Therefore, a conductive material made from a graphite-based material or a metal-based material is used as the separator, and in particular, those made from a metal-based material are considered to be advantageous since they are superior in mechanical strength and since they are formed into a thin-plate so as to make them light and compact. The separator made from a metal is generally formed by press forming a thin plate made of a metal material having a high anti-corrosion property, such as stainless steel and titanium alloy, into a plate having an irregular shape in its cross-section, and those having a surface with a plated noble metal such as gold and platinum are also used. The separator made from metal of this type has been proposed in Japanese Patent Application Laid-Open No. 10-241709.
In the separator made of metal, the portion formed into the shape having the irregular shape in its cross-section forms a current collector unit in which grooved gas passages and cooling medium passages and protruding portions contacting the gas diffusion electrode plates of the membrane electrode assembly are alternately formed continuously. Here, in the case when this current collector unit is formed through the press forming, the metal plate is subjected to bending and extending. A stress resulting from the press forming tends to occur inside thereof, resulting in a warp in the separator after the molding process. The separator having a warp causes problems in which the protruding portions are not allowed to contact the gas diffusion electrode plates with a sufficient surface pressure or in which the surface pressure becomes nonuniform, resulting in high contact resistance and a subsequent reduction in generated voltage. When the fuel cell stack is formed by stacking a plurality of the membrane electrode assemblies while interpolating the separators therebetween, the assembling process has to be carried out while correcting the warp; therefore, the operation is complicated and difficult, resulting in another problem of a reduction in gas sealing properties.